Organic light-emitting devices (OLEDs) have shown great potential for general illumination. A typical OLED consists of one or more organic layers sandwiched between two electrodes, which are stacked on a supporting substrate such as glass and plastic sheet. The OLED operates as an electroluminescent device. Light generated in the organic layers propagates in all the directions.
State-of-the-art OLED emissive materials generally have a refractive index greater than 1.7 that is substantially higher than that of most of the supporting substrates (usually ˜1.5). As light propagates from a higher index medium to a lower index medium, total internal reflection (TIR) occurs for light beams travelling in large oblique angles relative to the interface, according to Snell's law. TIR leads to power trapping in the higher index medium and reduces light extraction efficiency. In a typical OLED device, TIR occurs between organic layers (refractive index ˜1.7) and the substrate (refractive index ˜1.5); and between the substrate (refractive index ˜1.5) and air (refractive index 1.0), which leads to light trapping in the “organic modes” as well as the “substrate modes”. In addition, some of the emitted light is coupled to “surface plasmon mode”, which is a surface wave traveling along the metal cathode-ETL interface. This results in light trapping in the device and further reduces light extraction efficiency. Thus, an OLED, in its simplest form, usually exhibits poor light extraction efficiency. It is commonly believed that only ˜20% of light generated can escape from an OLED device without any light extraction mechanism.
Different technical methods and approaches have been used to improve light extraction. Examples include substrate surface roughening, surface texturing, such as 2D photonic structures, the use of microlenses, and the use of scattering films. These approaches have led to enhancements in light extraction through substrate modification and optimization. However, work in the field to date has mainly focused on substrate modification.
In dry-coated OLEDs, all the organic layers have similar refractive index. For wet-coated OLEDs, however, state-of-the-art OLEDs typically employ a p-doped polymeric hole-injection layer (HIL); a well-known example of a polymeric HIL is PEDOT: PSS. Most of the p-doped polymeric HILs have a refractive index of <1.5, which is much less than that of state-of-the-art OLED emissive materials (typically >1.7). As a result, additional TIR occurs at the EML-HIL interface and further reduces light extraction efficiency. For planarization of the anode, typical thickness of HIL is greater than 100 nm. However, as the thickness of HIL increases, light penetration through HIL decays further. For example, in a low index HIL, EQE decreases with increasing HIL thickness because of the TIR at the emissive layer and HIL interface, resulting in an additional 14% loss in emitted power. Therefore, the mismatch in refractive index is one of the limiting factors for light extraction efficiency in wet-coated OLEDs, and there is a need to improve light extraction from wet-coated OLED devices.